The invention relates to an energy-focusing and space-angle focusing reflector for time-of-flight mass spectrometers.
Two-stage reflectors with grids are known from the work of B. A. Mamyrin, V. I. Karatzev and D. V. Shmikk (U.S. Pat. No. 4,072,862). These allow ions to be reflected with velocity focusing with an adjustable focal length (see FIG. 1). An initial, strong opposing field decelerates the ions while a second, well homogenized field reflects the ions to produce the velocity focusing. The focal length of the energy focusing can be adjusted by adjusting the field-strength ratio of the deceleration and reflection field. This reflector does not produce space-angle focusing.
Instead of two-stage reflectors, single-stage reflectors with only one grid in the input area can be used; these have a fixed, relatively short focal length for energy focusing and take up a large proportion of the total flight path of the time-of-flight mass spectrometer.
The work of R. Frey and E. W. Schlag (EP 0 208 894, U.S. Pat. No. 4,731,532) discloses grid-free, two-stage reflectors which provide space-angle focusing as well as a velocity focusing (FIG. 2). The space-angle focusing is produced by the grid-free deceleration field, which acts like an ion lens. However, the focal lengths of the velocity focusing and the space-angle focusing cannot be adjusted independently from each other; only a certain geometric arrangement is able to form an image on an ion detector from a slightly divergent ion beam emerging from a single source which is both velocity focused and space-angle focused.
The grid-free reflector is assembled from a number of metallic annular electrodes and a terminating electrode. The terminating electrode is usually in the form of a grid so that the spectrometer can also be operated in non-reflecting, linear mode with an ion detector placed behind the terminating electrode. The first two annular electrodes can have a relatively small internal diameter. A strong deceleration field is set up between them by applying a high potential difference. The equipotential lines which emerge from the space between, and pass through, the electrode apertures form the space-angle focusing ion lens. The other annular electrodes have the same internal diameter, the same distances between them and the same potential differences. They form a homogeneous reflection field which provides the energy focusing for ions of different energies due to the different penetration depths (and therefore different flight paths). The focal length of the energy focus is adjusted by the ratio of the field strengths in the deceleration and the reflection fieldxe2x80x94as with the grid reflector. The focus adjustment of the space-angle focusing, the focal length of which is not normally the same as that of the energy focusing, is therefore permanently coupled.
The invention consists in producing a weaker electrical field strength in the final section of the reflector field component. This creates the conditions for a field penetration of the somewhat stronger fields of the previous sections into the region of the weaker field and, due to the field penetration, slightly curved equipotential surfaces in the area of the last annular electrode. If the ions which are injected into this region are now brought to a stop before they are accelerated in the opposite direction, they are slightly deflected by the curved potential surfaces as they are reflected. Marginal beams which do not pass along the axis are deflected toward the axis and are therefore space-angle focused. The degree of deflection, and therefore the focal length of the space-angle focusing, can be adjusted by the degree to which the equipotential surfaces are curved. If the degree to which the equipotential surfaces are curved is predetermined and fixed, then the focal length of the space angle focusing can be adjusted by the total voltage at the reflector, i.e. by the penetration depth of the ions into the area of the increasingly curved equipotential surfaces.
The weaker field in the final section can be produced by lower potential differences with the same distance between the electrodes, by a larger distance between the last annular electrode and the terminating electrode at the same potential difference, by a combination of the two or by a dented terminating electrode. The dented terminating electrode pulls the curved equipotential surfaces as far as the terminating electrode. The dent does not have to be curved, a simple recess is sufficient.
The invention can be used in both single-stage and two-stage reflectors, with or without grids in each case.